A human eye is a specialized sensory organ capable of light reception and is able to receive visual images. Aqueous humor is a transparent liquid that fills at least the region between the cornea, at the front of the eye, and the lens. A trabecular meshwork, located in an anterior chamber angle, which is formed between the iris and the cornea, normally serves as a drainage channel for aqueous humor from the anterior chamber so as to maintain a balanced pressure within the anterior chamber of the eye.
Glaucoma is a group of eye diseases encompassing a broad spectrum of clinical presentations, etiologies, and treatment modalities, Glaucoma causes pathological changes in the optic nerve, visible on the optic disk, and it causes corresponding visual field loss, resulting in blindness if untreated, Lowering intraocular pressure is a major treatment goal in glaucomas.
In glaucomas associated with an elevation in eye pressure (intraocular hypertension), a main source of resistance to outflow is typically in the trabecular meshwork. The tissue of the trabecular meshwork normally allows the aqueous humor (hereinafter also referred to as “aqueous”) to enter Schlemm's canal, which then empties into aqueous collector channels in the posterior wall of Schlemm's canal and then into aqueous veins, which form the episcleral venous system. Aqueous is continuously secreted by a ciliary body around the lens so there is a constant flow of aqueous from the ciliary body to the anterior chamber of the eye. Pressure within the eye is determined by a balance between the production of aqueous and its exit through the trabecular meshwork (major route) and uveoscleral outflow (minor route) pathways. The portion of the trabecular meshwork adjacent to Schlemm's canal (the juxtacanilicular meshwork) can cause most of the resistance to aqueous outflow.
Glaucoma is broadly classified into two categories: closed-angle glaucoma, also known as angle closure glaucoma, and open-angle glaucoma. Closed-angle glaucoma is caused by closure of the anterior chamber angle by contact between the iris and the inner surface of the trabecular meshwork. Closure of this anatomical angle prevents normal drainage of aqueous from the anterior chamber of the eye.
Open-angle glaucoma is any glaucoma in which the exit of aqueous through the trabecular meshwork is diminished while the angle of the anterior chamber remains open. For most cases of open-angle glaucoma, the exact cause of diminished filtration is unknown. Primary open-angle glaucoma is the most common of the glaucomas, and is often asymptomatic in the early to moderately advanced stages of glaucoma. Patients may suffer substantial, irreversible vision loss prior to diagnosis and treatment.
Most current therapies for glaucoma are directed toward decreasing intraocular pressure. Medical therapy includes topical ophthalmic drops or oral medications that reduce the production of aqueous or increase the outflow of aqueous. However, drug therapies for glaucoma are sometimes associated with significant side effects. The most frequent and perhaps most serious drawback to drug therapy, especially the elderly, is patient compliance. Patients often forget to take their medication at the appropriate times or else administer eye drops improperly, resulting in under- or overdosing. Patient compliance is particularly problematic with therapeutic agents requiring dosing frequencies of three times a day or more, such as pilocarpine. Because the effects of glaucoma are irreversible, when patients dose improperly, allowing ocular concentrations to drop below appropriate therapeutic levels, further permanent damage to vision occurs.